Satanism begins with Anton Szandor LaVey and the original Church of Satan. There is no better place to begin your inquiry.
Origin Of Satanism
Anton Szandor LaVey created Satanism and brought it to the public in 1966 CE. The Satanic Bible, published in 1969, is the primary and authoritative source for Satanism. Currently, The Satanic Bible is only available through a single publisher. Do ensure that you are purchasing a legitimate copy when you are able. Many brick-and-mortar stores still carry the title and is always available on Amazon.
Satanism explicitly holds the highest regard for individualism. There are no political protests, statues, churches, public covens, grottos, or remote charter memberships. If you believe you have received notification suggesting otherwise then you have confused another organization with Church of Satan.
Anton Szandor LaVey did not claim to have any spiritual communication or Gnosticism. Satanism's foundation is rational thought. Antiquated dogma has no place in the rational mind. LaVey stresses that it is up to the individual to know themselves and all mankind as carnal animals. Personal exploration through unclouded rational thought is critical for a Satanist.
Practice of LeVayan Satanism can be accompanied by officially joining Church of Satan. Individuals are free to practice alone with LaVey's original texts and with media produced by Church of Satan members. There is no requirement to join to be a Satanist.
Be wary of the many groups that claim to be Satanists.
If a person requests money or action from you on behalf of Church of Satan it is most likely fraudulent. You should contact Church of Satan if you are concerned.
Concepts
Satanism Existed Before LaVey
This is only true in the most naive of considerations.
Historically, people may use words like heresy and blasphemy to accuse and shame individuals of not being of the normative in behavior and religious practice. The word satanic has been penned by many an overzealous fanatic as a rallying cry to the faithful. Popular media may suggest that red-eyed satanic demons flying a flag of Baphomet fought in dark battlegrounds. These battles are cogs in the imaginations of individuals inventing enemies for the heroes they wish to identify as and nothing more. Historical battles took place under the names of invented beings "guiding" warriors into battle to defeat individuals that did not believe in the same invented being - Satanism was not involved. Satanists have never sieged a castle. It was particularly difficult to find a castle to siege in California circa 1966 CE and it hasn't become any easier since.
Satanism Is Inherently Evil
Any theistic religion has definitions for good and evil, what is right, and what an individual may claim dominion over. Luckily, Satanism is not a theistic religion. There are no mysterious texts from unknown sources dictating the behavior of its members. There are no tithes or confessionals. No mandate requires you to reject any thought nor does it require any unquestionable action. The individual that identifies with Satanism chooses appropriately through review and introspection.
Other Groups
Pseudos
Pseudos or pseudo satanists is a mature term used for a few decades to denote individuals that are attracted to the media, attention, and imagery of Satanism but have chosen their own unique twist of Satanism.
First Satanic Church
Karla LaVey, daughter of Anton LaVey, founded the First Satanic Church as a continuation of her father's legacy.
Temple of Set
Founded by Michael Aquino, previously a high ranking member of the original Church of Satan, the Temple of Set splintered off into a new direction in 1975. Dissatisfied with LaVeyan Satanism, Aquino founded this organization based on mystical knowledge he received from Set.
The Satanic Temple
Unlike LaVeyan Satanism, The Satanic Temple is an outgoing organization involved in widely public initiatives. Founded in 2014, The Satanic Temple uses imagery and "Satanism flair" with the core principles being quite different from LaVeyan Satanism.
If you hear of activists for Satanism or statues representing Satanism you are most likely reading about this group. There is no relation between Church of Satan and The Satanic Temple.
Various Satanic/Satanism Groups
Many groups may offer a "theistic Satanism", "tantric Satanism", or alternative flavor of Satanism and definitely count as pseudos. Perhaps they will sculpt a new philosophy and decorate it with images and iconography from Satanism to brand their group. Anton Szandor LaVey defined Satanism in 1966 and Church of Satan continues to present day. You are, of course, free to feel you identify with these groups and not with LaVeyan Satanism, but do keep in mind the age and longevity of Satanism and that it begins with Anton Szandor LaVey.